1. Field of Invention
The invention relates to an exhaust emission control apparatus and a control method of an internal combustion engine.
2. Description of Related Art
There is a known internal combustion engine for combustion of fuel at a lean air/fuel ratio having a NOX catalyst disposed within an exhaust passage. The NOX catalyst stores NOX contained in exhaust gas flowing into the NOX catalyst at a lean air/fuel ratio, and reduces the stored NOX under the presence of a reducing agent contained in the exhaust gas upon decrease in the air/fuel ratio. The aforementioned internal combustion engine further includes a bypass passage that extends to branch off from the exhaust passage upstream of the NOX catalyst, and a bypass control valve that serves to adjust a flow rate of the exhaust gas flowing into the bypass passage so as to control the flow rate of the exhaust gas flowing through the NOX catalyst. A reducing agent supply valve through which the reducing agent is supplied to the NOX catalyst is disposed within the exhaust passage between the point where the bypass passage is branched and the NOX catalyst. In the above-structured internal combustion engine, the flow rate of the exhaust gas flowing through the NOX catalyst is temporarily decreased by the bypass control valve, and at the same time, the reducing agent is supplied from the reducing agent supply valve.
The above structure may decrease the quantity of the reduction agent which is required to set the air/fuel ratio of the exhaust gas flowing into the NOX catalyst to the rich or the theoretical state by decreasing the flow rate of the exhaust gas upon supply of the reducing agent through the reducing agent supply valve. As the space velocity of the exhaust gas within the NOX catalyst is decreased, the quantity of the reducing agent flowing through the NOX catalyst without causing reaction can be decreased, resulting in efficient use of the reducing agent.
The above-structured internal combustion engine controls the bypass control valve such that the flow rate of the exhaust gas flowing into the NOX catalyst sequentially changes from the timing when the flow rate begins decreasing until it resumes the originally set value. The reducing agent may be efficiently used at an optimum flow rate of the exhaust gas flowing through the NOX catalyst upon supply of the reducing agent through the reducing agent supply valve. It is, therefore, preferable to determine the timing at which the flow rate of the exhaust gas flowing through the NOX catalyst becomes the optimum value for the efficient use of the reducing agent. This makes it possible to supply the reducing agent through the reducing agent supply valve at the determined timing.
Each of the bypass control valves, however, widely varies in terms of performance. This may cause the flow rate of the exhaust gas flowing through the NOX catalyst to become larger or smaller than the optimum value even if the reducing agent is supplied at the determined timing.